Silica and Alumina
Explanation
The Earth's crust is the outermost solid shell of the planet. It is classified into two distinct types based on chemical composition, density, and thickness: the continental crust and the oceanic crust. These layers are historically distinguished by the acronyms SIAL and SIMA.
Option Analysis
- Silica and Magnesium is Incorrect: Silica and Magnesium are the primary constituents of the oceanic crust. This layer is referred to as SIMA. It is generally thinner (approx. 5 km) and denser than the continental crust.
- Silica and Alumina is Correct: The continental mass (continental crust) mainly consists of Silica and Alumina. It is referred to as SIAL (Si for Silica and Al for Alumina). The continental crust is significantly thicker (approx. 35 km) and less dense than the oceanic crust.
- Nickel and Iron is Incorrect: Nickel and Iron are the main components of the Earth's core (both outer and inner core). This layer is known as NIFE (Ni for Nickel and Fe for Ferrous/Iron).
- Iron and Alumina is Incorrect: Iron and Alumina is not a standard classification for the primary composition of any major layer of the Earth's interior.
Key Takeaway:
The chemical composition of Earth's layers is categorized as SIAL (Silica + Alumina) for the Continental Crust, SIMA (Silica + Magnesium) for the Oceanic Crust, and NIFE (Nickel + Iron) for the Core.